Charging System for an Implantable Medical Device Employing Magnetic and Electric Fields

ABSTRACT

A base station for passively recharging a battery in an implant without patient involvement is disclosed. The base station can be hand held or may comprise equipment configured to be placed at a fixed location, such as under a bed, on or next to a wall, etc. The base station can generate electric and magnetic fields (E-field and B-field) that couple with an antenna and a receiving coil within the implant to generate a charging current for charging the implant&#39;s battery. No handling or manipulation on part of the patient is necessary; the implant battery is passively charged whenever the patient is within range of either the magnetic or electric charging fields generated by base station. Charging using the B-field occurs when the IPG is at a relatively short distance from the base station, while charging using the E-field occurs at longer distances.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.13/164,005, filed Jun. 20, 2011, which is a non-provisional applicationof U.S. Patent Application Ser. No. 61/360,536, filed Jul. 1, 2010.Priority is claimed to these applications, and they are incorporatedherein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to improved battery charging for animplantable medical device.

BACKGROUND

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder sublaxation, etc.The description that follows will generally focus on the use of theinvention within a Spinal Cord Stimulation (SCS) system, such as thatdisclosed in U.S. Pat. No. 6,516,227. However, the present invention mayfind applicability in any implantable medical device system.

As shown in FIGS. 1A and 1B, a SCS system typically includes anImplantable Pulse Generator (IPG) 100, which includes a biocompatibledevice case 30 formed of a conductive material such as titanium forexample. The case 30 typically holds the circuitry and battery 26necessary for the IPG to function, although IPGs can also be powered viaexternal RF energy and without a battery. The IPG 100 includes one ormore electrode arrays (two such arrays 102 and 104 are shown), eachcontaining several electrodes 106. The electrodes 106 are carried on aflexible body 108, which also houses the individual electrode leads 112and 114 coupled to each electrode. In the illustrated embodiment, thereare eight electrodes on array 102, labeled E₁-E₈, and eight electrodeson array 104, labeled E₉-E₁₆, although the number of arrays andelectrodes is application specific and therefore can vary. The arrays102, 104 couple to the IPG 100 using lead connectors 38 a and 38 b,which are fixed in a non-conductive header 36, which can comprise anepoxy for example.

As shown in FIG. 2, the IPG 100 typically includes an electronicsubstrate assembly 14 including a printed circuit board (PCB) 16, alongwith various electronic components 20, such as microprocessors,integrated circuits, and capacitors mounted to the PCB 16. Two coils(more generally, antennas) are generally present in the IPG 100: atelemetry coil 13 used to transmit/receive data to/from an externalcontroller 12; and a charging coil 18 for charging or recharging theIPG's battery 26 using an external charger 50. The telemetry coil 13 istypically mounted within the header 36 of the IPG 100 as shown, and maybe wrapped around a ferrite core 13′.

As just noted, an external controller 12, such as a hand-held programmeror a clinician's programmer, is used to wirelessly send data to andreceive data from the IPG 100. For example, the external controller 12can send programming data to the IPG 100 to dictate the therapy the IPG100 will provide to the patient. Also, the external controller 12 canact as a receiver of data from the IPG 100, such as various datareporting on the IPG's status. The external controller 12, like the IPG100, also contains a PCB 70 on which electronic components 72 are placedto control operation of the external controller 12. A user interface 74similar to that used for a computer, cell phone, or other hand heldelectronic device, and including touchable buttons and a display forexample, allows a patient or clinician to operate the externalcontroller 12. The communication of data to and from the externalcontroller 12 is enabled by a coil (antenna) 17.

The external charger 50, also typically a hand-held device, is used towirelessly convey power to the IPG 100, which power can be used torecharge the IPG's battery 26. The transfer of power from the externalcharger 50 is enabled by a coil (antenna) 17′. For the purpose of thebasic explanation here, the external charger 50 is depicted as having asimilar construction to the external controller 12, but in reality theywill differ in accordance with their functionalities as one skilled inthe art will appreciate.

Wireless data telemetry and power transfer between the external devices12 and 50 and the IPG 100 takes place via inductive coupling, andspecifically magnetic inductive coupling. To implement suchfunctionality, both the IPG 100 and the external devices 12 and 50 havecoils which act together as a pair. In case of the external controller12, the relevant pair of coils comprises coil 17 from the controller andcoil 13 from the IPG 100. In case of the external charger 50, therelevant pair of coils comprises coil 17′ from the charger and coil 18from the IPG 100.

When data is to be sent from the external controller 12 to the IPG 100for example, coil 17 is energized with an alternating current (AC). Suchenergizing of the coil 17 to transfer data can occur using a FrequencyShift Keying (FSK) protocol for example, such as disclosed in U.S.Patent Application Publication 2009/0024179. Energizing the coil 17produces a magnetic field, which in turn induces a voltage in the IPG'scoil 13, which produces a corresponding current signal when provided aclosed loop path. This voltage and/or current signal can then bedemodulated to recover the original data. Transmitting data from the IPG100 to the external controller 12 occurs in essentially the same manner.

When power is to be transmitted from the external charger 50 to the IPG100, coil 17′ is again energized with an alternating current. Suchenergizing is generally of a constant frequency, and may be of a largermagnitude than that used during the transfer of data, but otherwise thebasic physics involved are similar.

The IPG 100 can also communicate data back to the external charger 50 bymodulating the impedance of the charging coil 18. This change inimpedance is reflected back to coil 17′ in the external charger 50,which demodulates the reflection to recover the transmitted data. Thismeans of transmitting data from the IPG 100 to the external charger 50is known as Load Shift Keying (LSK), and is useful to communicate datarelevant during charging of the battery 26 in the IPG 100, such as thecapacity of the battery, whether charging is complete and the externalcharger can cease, and other pertinent charging variables. LSKcommunication from an IPG 100 to an external charger is discussedfurther in U.S. Patent Application Publication 2010/0179618.

As is well known, inductive transmission of data or power can occurtranscutaneously, i.e., through the patient's tissue 25, making itparticularly useful in a medical implantable device system. During thetransmission of data or power, the coils 17 and 13, or 17′ and 18,preferably lie in planes that are parallel, along collinear axes, andwith the coils as close as possible to each other. Such an orientationbetween the coils 17 and 13 will generally improve the coupling betweenthem, but deviation from ideal orientations can still result in suitablyreliable data or power transfer.

Although the burden on the patient to charge the IPG seems minimal, theinventors recognize that some percentage of the patient population doesnot have the skills necessary to operate the charger 50. For example,some patients may be physically impaired and thus unable to place acharger 50 at the appropriate location over the IPG 100. Furthermore,even in patients that are able, it may be difficult for the patient totell where the IPG 100 is located, or what an appropriate alignmentwould be between the charger 50 and the IPG 100. In short, the need forthe patient's involvement in the charging process can be problematic,and the inventors here introduce a solution that can allow patients torecharge their implants with no or little participation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show an implantable medical device, and the manner inwhich an electrode array is coupled to the IPG in accordance with theprior art.

FIG. 2 shows the relation between the implantable medical device, anexternal controller, and an external charger.

FIG. 3 shows the addition of a base station charger to the system ofFIG. 2.

FIGS. 4A and 4B show the base station charging an IPG using an E-fieldand a B-field, respectively, in accordance with an embodiment of theinvention.

FIGS. 5A-5E depict various physical embodiments of base station of FIG.3.

FIG. 6 shows an IPG electrode being used as an antenna for E-fieldreception in accordance with an embodiment of the invention.

FIG. 7 shows a schematic of the circuitry within the base station inaccordance with an embodiment of the invention.

FIGS. 8A and 8B show schematics of the circuitry within the IPG forinterfacing with the base station in accordance with an embodiment ofthe invention.

FIGS. 9 and 10 show alternative schematics of a base station and the IPGhaving an additional communication channel and hardware.

DETAILED DESCRIPTION

The description that follows relates to use of the invention withinspinal cord stimulation (SCS) system. However, it is to be understoodthat the invention is not so limited, and could be used with any type ofimplantable medical device system.

The inventors address the problem of recharging a battery in an implantby providing a external device that can passively recharge the batterywithout patient involvement. The external device is referred to as abase station 200, and is shown in conjunction with a traditionalexternal controller 12 and external charger 50 in FIG. 3. The basestation 200 can be hand held similar to devices 12 or 50, but in thedisclosed embodiments is described as equipment configured to be placedat a fixed location, such as under a bed, on or next to a wall, etc. Inother words, the base station 200 would normally be located somewherewhere the patient would be expected to spend a significant amount oftime—time which can be spent recharging the battery. The base station200 could be battery-powered, but would more likely be plugged into awall socket.

Base station 200 in one embodiment can generate an electric field and amagnetic field (E-field and B-field) that couple with an antenna and areceiving coil within the IPG 100 to generate a charging current forcharging the IPG battery 26. No handling or manipulation on part of thepatient is necessary; the implant battery is passively charged wheneverthe patient is within range of either the magnetic or electric chargingfields generated by base station 200. Charging using the B-field occurswhen the IPG is at a relatively short distance from the base station 300(e.g., less than 1 m), while charging using the E-field occurs at longerdistances (e.g., >1 m). Back telemetry from the IPG 100 to the basestation 200 can be used to inform the base station 200 as to whethercharging should occur via B-field or E-field, and B-field charging ispreferred if possible for its ability to transfer higher amounts ofpower to the IPG 100, as will be explained here.

FIGS. 4A and 4B illustrate both E-field and B-field modes of operationof the base station 300. FIG. 4A shows base station 200 using an antenna204 for generating a radiating E-field 302. The E-field 302 is sensed byan antenna 150 in IPG 100 to generate an alternating current, which isrectified to produce DC power to recharge the battery, as will bedescribed in further detail later. Because these antennas 204 and 150primarily interact with the electrical component of the electromagneticfield, FIG. 4A illustrates only the E-field 302. FIG. 4B shows the basestation 200 using a coil (antenna) 206 for generating an inductiveB-field 304. Coil 18 in IPG 100 couples with the B-field 304 to generatean alternating current, which is rectified to produce DC power. SuchB-field charging is similar to the charging scheme implemented in atraditional external charger 50 (FIG. 2), and uses similar circuitry,although circuitry in the base station 302 has been modified asdiscussed herein. For example, the base station's circuitry allows forselection of the charging mode—E-field 302 or B-field 304—to transferenergy to the IPG 100. As discussed further below, the B-field 304 istypical a lower frequency (e.g., on the order of 100 kHz) than theE-field 302 (e.g., on the order of 1 MHz to 10 GHz).

As noted, and as depicted in FIG. 4, E-field charging will be used forlonger distances, while B-field charging will be used for shorterdistances. The strength of an E-field, such as E-field 302, typicallyreduces proportional to the square of the distance between thetransmitting antenna 204 and the receiving antenna 150. In contrast,strength of an inductive magnetic field, such as B-field 304, typicallyreduces proportional to the cube of the distance between the generatingcoil 206 and the receiving coil 18. Therefore, for larger distances,transferring energy using an E-field is more efficient that using aB-field.

Before discussing the circuitry and operation of the base station 200,various physical embodiments of base station 200 are discussed, as shownin FIGS. 5A-5E. As noted earlier, base station 200 can be positioned ona wall or floor, i.e., placed near a bed, against a wall, in a corner,or at any other convenient location generally close to an expectedlocation of a patient. FIG. 5A shows base station 200 having aserpentine wire antenna 204 connected to a circuit module 208. As willbe discussed later, circuit module 208 includes circuitry such asmicrocontrollers, amplifiers, transceivers, etc., for operating basestation 200. Serpentine antenna 204 is energized by the circuit module208 for radiating the E-field 302. Alternatively, an inductively-loadedantenna such as the one shown in FIG. 5B can be used in place of theserpentine antenna. Generally, both the antennas 204 illustrated inFIGS. 5A and 5B would be quarter-wavelength monopole antennas. Aquarter-wavelength antenna ideally has a length equal to one-fourth ofthe wavelength of the E-field being radiated. For example, aquarter-wavelength antenna 204 of length 0.25 m would be used fortransmitting a wavelength of 1 meter (which corresponds to a frequencyof approximately 300 MHz). Because of the serpentine shape of theantenna of FIG. 5A, and because of the inductive loading of the antennaof FIG. 5B, these antennas can be made smaller than the optimalquarter-wavelength length. Base station 200 can be equipped with aparabolic reflector (not shown) placed behind the antenna 204 to radiateor propagate energy of the E-field in the desired direction. This can beof particular advantage to focusing the radiating E-field towards thepatient's IPG 100.

Base station 200 also includes the coil 206 for generating the B-field304, which coil is also coupled to the circuit module 208. Coil 206 istypically wound on a ferrite core (not shown) to increase the strengthof produced inductive field.

Because the antennas 204 of FIGS. 5A and 5B are vertically oriented, theelectromagnetic wave radiated from the antenna 204 is also verticallyoriented, i.e., vertically polarized. If the IPG antenna 150 is alsovertically oriented, maximum coupling with the vertically orientedE-field 302 will occur. Maximum coupling is favorable because it resultsin the maximum E-field power transfer, thus providing more energy torecharge the IPG 100's battery 26. Such coupling will diminish as theangle between the polarization of the E-field 302 and the orientation ofthe receiving IPG antenna 150 increases, with minimum coupling at a90-degree angle.

FIG. 5C shows another embodiment of a base station 200 that includes apatch antenna 204 coupled to the circuit module 208. Patch antenna 204is typically made of a square or rectangular metal plate placed at acertain distance over a ground plane, which may comprise an additionalmetal plate connected to ground or the floor itself. Patch antennasgenerally operate as dipole, or half-wavelength antennas, meaning thatthe antenna is ideally dimensioned to half the wavelength of thetransmitted electromagnetic wave. For example, if the patch antenna 204is used to generate an E-field 304 at 300 MHz, the length of patchantenna would ideally be equal to 0.5 m. Patch antenna 204 of FIG. 5Cwill radiate or propagate energy of an E-field vertically upwards, i.e.,in a direction that is generally normal to the plane of the patchantenna, and is therefore useful for placement underneath a patient'sbed for example to allow for IPG charging while the patient is sleeping.Polarization of the E-field generated by the patch antenna is determinedby the location of the contact(s) 210 as shown in FIG. 5C, which is thelocation where the patch antenna is coupled to the circuit module 208.Again, maximum coupling between the E-field generated by the patchantenna 204 and the IPG antenna 150 will occur when the direction of theIPG antenna 150 is the same as the direction of the polarization of theE-field. Note that the base station of FIG. 5C can also be placedvertically, as shown in FIG. 5D. Such orientation could be mounted on awall, and might be more advantageous to recharge an IPG in a patientsitting in a nearby chair for example. Antenna 204 can also comprise aslot antenna.

FIG. 5E shows an embodiment of base station 200 that combines theembodiments shown in FIGS. 5C and 5D, and thus provides both horizontaland vertical polarization of the produced E-field. Because the producedE-field is polarized in two directions, it will more likely couple tothe antenna 150 in the IPG 100, which antenna 150 orientation may not beknown exactly or can vary as the patient moves. In this embodiment, thebase station 200 includes two patch antennas 204 a and 204 brespectively placed in horizontal and vertical planes, allowing theenergy of E-field to be radiated or propagated in both upward andsideways directions. Patch antenna 204 a also includes two contactpoints 210 a and 210 c, which allows the base station 200 to select thedesired polarization. Patch antenna 204 a can be simultaneouslyenergized at both contact points 210 a and 210 c to generate acircularly-polarized E-field, typically by energizing points 210 a and210 c 90-degrees out of phase. Such circularly polarized field minimizesconstraints on the orientation of the IPG antenna 150 for maximumcoupling. Patch antenna 204 b may likewise contain two contact pointsand produce a circularly-polarized E-field, although this is not shownin FIG. 5E for clarity.

The base station 200 of FIG. 5E also contains two cols 206 a and 206 b.Like the antennas 204 a and 204 b, the coils 206 a and 206 b areorthogonal, and produce B-fields which are orthogonal, which minimizesconstraints on the orientation of the IPG coil 18 (FIG. 4B). A rotatingB-field can also be produced using the two coils 206 a and 206 b. See,e.g., U.S. Patent Application Publication 2009/0069869, which isincorporated herein by reference in its entirety.

Base station 200 can also select the antenna 204 a or 204 b, or coil 206a or 206 b, that provides maximum power transfer to the IPG 100, and useonly that antenna or coil. This selection can be based on assessingcoupling information for each antenna and coil orientation, whichinformation can be telemetered from the IPG 100, or can be deduced basedon the production of the E-field or B-field at the base station 200.See, e.g., U.S. Patent Application Publication 2008/0172109, which isincorporated herein by reference.

FIG. 6 shows further details of a suitable E-field antenna 150, and inthis embodiment antenna 150 comprises one of the electrode leads 112used in array 102 as the antenna 150. For example, the wire connectingto electrode E₁ is used as the antenna 150. Wires to other electrodes(E2, E3) can also be used, but because selecting the longest wireadvantageously reduces the transmission frequency, the wire to electrodeE₁ has been chosen. (Of course, a signal wire connecting to electrodeson array 104 can also be chosen). Because electrode leads 112 and 114provide individual wires of varying lengths, a wire whose length isclosest to the ideal length for a particular E-fieldreceiving/transmitting frequency can be readily selected. Note thatusing an electrode lead for the antenna 150 does not affect stimulationproduced at the affected electrode because the frequency of the E-field302 received or transmitted by antenna 150 is at least a few orders ofmagnitude higher than the frequency of signals sent to electrodes. Forexample, the E-field 302 is typically on the order of 1 MHz to 10 GHz,while the frequencies of pulses sent to the electrode via a signal wireare in the range of tens of Hz to hundreds of Hz. Moreover, themagnitude of the AC signal on the signal wire resulting from E-fieldtransmission or reception is typical very small (e.g., mV) compared tothe magnitude of the stimulation pulses (e.g., Volts). Of course, theIPG 100 can also include a dedicated antenna, separate from theelectrode leads 112 and 114, for transmitting and receiving the E-field302 to and from the base station 200. Such an antenna can be placed inthe header 36 or in the metal case 30 of the IPG 100.

Discussion now turns to the circuit module 208 in the base station 200used to transfer and receive energy to and from the IPG 100. As shown inFIG. 7, microcontroller 212 controls the operation of the transmissioncircuitry and receiver circuitry, as well as controlling otheroperations in the base station 200 not discussed here. As is typical,microcontroller 212 can include both volatile memory (e.g., RAM) andnon-volatile memory (e.g., Flash, EEPROM) for storing and implementingthe functionality described herein. Transmission circuitry includes adigitally-controlled signal generator 214 and power amplifier 216.Receiver circuitry includes two receiver circuits, LSK Rx 220 and RF Rx228. Switch 222 couples the transmission and receiver circuitry toeither the antenna 204 or the coil 206, depending on whether E-field orB-field charging has been selected.

For transferring energy using coil 206 via B-field 304, microcontroller212 controls the signal generator 214 to generate a signal with atransmission frequency of f_(B)=80 kHz for example. Signal generator 214will typically generate a sinusoidal signal at the specified frequency,but can also generate waveforms with a varying duty-cycle.

For transferring energy using antenna 204 via E-field 302,microcontroller 212 controls the signal generator to generate a signalwith a transmission frequency of f_(E). f_(E) can range from about 1 MHzto 10 GHz, and whether higher or lower frequencies are used for f_(E)involve tradeoffs. Transmitting at higher frequencies allows higherenergy to be transmitted to the IPG 200, and at longer distances.However, high frequency signals are attenuated by the body tissue. Lowerfrequencies have less attenuation, but can require a longer antenna 150in the IPG 100 for optimal quarter-wavelength tuning Antenna length ismitigated slightly by the permittivity of the tissue, which is primarilywater. Because the length of the antenna 150 will scale in inverseproportion to the square root of the permittivity of the tissue (water),the required length of the antenna 150 can be shortened significantly,which will allow f_(E) to be lowered. In any event, a lack of precisetuning and the reality of signal attenuation can be mitigated by properantenna circuitry design and by adjusting the power of the E-fieldtransmission, and it is not strictly necessary that an antenna 150 inthe IPG 100 be exactly one-quarter of the wavelength of f_(E). In usefulembodiments, f_(E) can comprise a frequency selected from theIndustrial, Scientific, and Medical (ISM) band in one example, and couldcomprise frequencies of 13.56 MHz, or 27.12 MHz, or 2.45 GHz forexample, even if the antenna 150 in the IPG 100 is not dimensioned toresonate optimally at such frequencies.

The output of signal generator 214 is fed to the input of poweramplifier 216, which amplifies its input signal by a magnitudecontrolled by the microcontroller 212 using a gain control signal. Inreality, separate amplifiers 216 may be used depending on the frequency(f_(E) or f_(B)) chosen, but this is not shown in FIG. 7 for simplicity.Initially, the microcontroller 212 may set a default gain for poweramplifier 216 via the gain control signal, which signal can be increasedas necessary.

The output of the power amplifier 216 is ultimately sent to either ofantennas 204 or coil 206 via appropriate impedance matching circuitry218 and 230. Impedance matching circuits are well known in the art, andcan include transformers, passive RLC networks, stepped transmissionlines, etc. Which of the antenna 204 or coil 206 are chosen isdetermined by a control signal K1 issued from the microcontroller 212,which equals a logic ‘1’ when B-field charging is used, and a logic ‘0’when E-field charging is used. When K1=1, switch 222 couples thetransmission and receiver circuitry to coil 206 via its impedancematching circuitry 230. When K1=0, switch 222 couples the circuitry toantenna 204 via its impedance matching circuitry.

In the embodiment of FIG. 7, base station 200 includes two receivercircuits for receiving back-telemetry data from the IPG 100 duringrecharging of the IPG battery 26. LSK receiver 220 receivesload-shift-keyed data via coil 206, while RF receiver 228 receivesmodulated data via antenna 204. Like switch 222, these receivers 220 and228 are controlled by K1, such that only one of them is enabled at atime, depending on whether the base station is operating in B-field orE-field mode. LSK telemetry is well known, and involves modulating theresistance of the receiving coil 18 in the IPG to produce detectablereflections at the transmitting coil 206, as is explained further belowwith reference to FIG. 8A.

Charging information back telemetered from the IPG 100 can include theIPG's battery voltage (V_(BAT)) and data indicative of the couplingbetween the base station and the IPG. V_(BAT) informs themicrocontroller 212 of the present voltage of the IPG battery 26 duringcharging to allow the microcontroller 212 to either modify the power ofthe antenna 204 or coil 206 broadcasting the charging energy, or tosuspend charging altogether once the battery 26 is full.

Coupling data received from the IPG 100 indicates the amount of energythat the IPG is receiving, and will depend upon several factors, such astransmission power, distance between the base station 200 and the IPG100, relative orientations of the transmitting/receiving elements(antenna 204 and antenna 150; or coil 206 and 18), etc. In oneembodiment, coupling data can comprise the voltages V_(DCE) and V_(DCB)respectively output by the B-field and E-field rectifiers 154 and 164 inthe IPG 100, as will be discussed later with reference to FIG. 8A. Inanother embodiment, coupling data can comprise a voltage drop acrosscharging circuitry 156 (FIG. 8A). See, e.g., U.S. Pat. No. 8,744,592,which is incorporated herein by reference in its entirety. When the basestation 200 receives such coupling data during charging, it can controlthe gain of power amplifier 216 via the gain control signal. Forexample, if the output voltage of rectifiers 154 or 164 (FIG. 8A),V_(DCE) or V_(DCB), in the IPG 100 reduces below a predetermined value,microcontroller 212 can increase the gain of power amplifier 216 so thatthe magnitude of the produced E-field 302 or B-field 304 increases. Howto adjust the gain control signal for a particular received value of thecoupling data can be determined by experimentation or simulation, andcan be stored as a look up table in memory associated with themicroprocessor 212.

FIG. 8A shows an embodiment of the circuitry in the IPG 100 forreceiving the charging energy broadcast by the base station 200, and forback telemetering charging information to the base station 200. Antenna150, which again can comprise one of the signal wires as discussedearlier with respect to FIG. 6, is coupled to a multiplier and rectifier164 through an impedance matching circuit 168, and receives the E-field302 generated by the base station 200. The rectifier 164 generates a DCvoltage, V_(DCE), which is used to charge the battery 26. FIG. 8Billustrates an example circuit that can be used for rectifier 164, whichis known in the art as a half-wave series multiplier or a Villardcascade. The rectifier 164 comprises a number of capacitor-diode stages,with four such stages shown in FIG. 8B. The number of stages dictatesthe multiplier that will be applied to the AC input voltage, Vin, toproduce DC voltage V_(DCE), such that four stages will essentiallyproduce a V_(DCE) that is four times the peak voltage of Vin. Diodes174-177 are preferably zero threshold or low threshold diodes such asSchottky diodes, which will allow for the rectification andmultiplication of small AC voltage, Vin, produced at the output of theantenna 150 (tens to hundreds of mVs). V_(DCE) is fed to the chargingcircuit 156, which monitors and controls the battery's 26 chargingprocess.

Referring again to FIG. 8A, IPG 100 also includes a charging coil 18connected to a rectifier 154 via an impedance matching circuit 152. Thiscoil 18 receives the B-field 304 generated by the base station 200. Coil18 may also receive a B-field from a more traditional external charger50, such as was discussed in FIG. 2, and in this regard, the improvedcircuitry of FIG. 8A does not disrupt the use of such legacy systemdesigns. Impedance matching circuit 152 matches the impedance of thecoil 18 with the input impedance of the rectifier 154 to allow formaximum power transfer. Rectifier 154 can be a single diode half-waverectifier, a full-wave bridge rectifier, or other rectifiers well knownin the art. Because the AC voltages induced on the coil 18 by theB-field are generally quite large (on the order of Volts), the rectifiercan use traditional diodes. Output of the rectifier 154, V_(DCB), is fedto the charging circuit 156.

Both V_(DCE) and V_(DCB) are fed to a comparison circuit 223 to becompared to threshold voltages V_(thE) and V_(thB), respectively.Generally speaking, comparison circuit 223 informs the microcontroller158 when charging energy is being received either at the antenna 150(via E-field charging) or at the coil 18 (via B-field charging). Asshown, comparison circuit 223 can include two comparators for comparingthe DC voltages produced by each of the rectifiers 164 and 154, V_(DCE)and V_(DCB), to reference voltages V_(thE) and V_(thB). If either DCvoltage exceeds its associated reference voltage, its comparator willdigitally indicate that fact to the IPG 100's microcontroller 158 as alogic ‘1’ at inputs X and Y. V_(thE) and V_(thB) can be experimentallydetermined, and can be made adjustable, but in any case are generallyset to a significant level to discern true power reception from merenoise. Note that because V_(DCE) will often be much less than V_(DCB),reference voltage V_(thE) will likewise generally be much smaller thanV_(thB). In an alternative arrangement, if the microcontroller 158includes or is associated with analog-to-digital converters, thenV_(DCE) and V_(DCB) can be directly fed to such analog inputs, allowingthe microprocessor 158 to assess the magnitude of those voltagesdigitally.

Microcontroller 158 can interpret input signals X and Y and issuecontrol signals B and E accordingly, which control signals indicate tothe remainder of the circuitry whether the microcontroller 158 isrecognizing and allowing charging of the battery 26 to occur via B-fieldor E-field reception. The following truth table shows the generation ofthese control signals B and E based on input signals X and Y, and showsthe preference of the IPG 100 to charge via B-field reception if thatroute is available.

X Y B E 0 0 0 0 0 1 0 1 1 0 1 0 1 1 1 0Allowing B-field charging to take precedence over E-field charging(i.e., B=1 when X is asserted, regardless of Y) is preferred because therectified voltage produced via B-field reception, V_(DCB), wouldgenerally be much greater than the rectified voltage produced viaE-field reception, V_(DCE). Allowing charging circuitry 156 to thenchoose V_(DCB) over V_(DCE) as its input voltage will allow suchcircuitry 156 to charge the battery 26 faster. Conversely, and asdiscussed further below, charging of the battery using V_(DCE) is usedas a last resort, and can occur passively. Charging circuitry 156 iswell known in the art, and is capable of handling input voltages ofdifferent values, such as would be provides by V_(DCE) and V_(DCB).Although shown as comprising two different inputs to charging circuitry156, it should be understood that V_(DCE) and V_(DCB) can be selected asa single input to the circuitry 156 using a switch controlled by controlsignals B and E (not shown). Of course, assertion of neither of controlsignals B or E would signify that the IPG 100 is not recognizing thereceipt of any charging field from the base station 200 (or any othersource such as the external charger 50), and will behave accordingly.

As discussed previously, the IPG 100 can back telemeter to the basestation 200 charging information, such as the battery voltage (V_(BAT))and coupling data, and such telemetry can also be controlled via controlsignals B and E. In this regard, and as shown in FIG. 8A, IPG 100contains a RF transmitter/receiver 166 enabled by control signal E, andan LSK transmitter 160 enabled by control signal B. In other words, theIPG 100 decides through this scheme to communicate back to the basestation 200 using the means (B-field or E-field) already established asreliable at the IPG 100 based on the fields it has received. LSKtransmitter 160, if chosen using control signal B, uses the charginginformation to be telemetered to modulate a transistor 168 connected inparallel with coil 18. As noted earlier, this produces reflection in thecoil 206 used in the base station to produce the B-field 304, which datacan then be demodulated at the LSK receiver 220 in the base station(FIG. 7) to recover the charging information. Should the RFtransmitter/receiver 166 be chosen via control signal E, the charginginformation will be modulated using a protocol suitable for broadcastvia the E-field antenna 150, such frequency shift keying (FSK), phaseshift keying (PSK), amplitude shift keying (ASK), etc. Such RF backtelemetered data would then be received at the RF receiver 228 in thebase station 200 (FIG. 7). Circuit 166 can also include correspondingdemodulation circuits for receiving data from the base station 200, andin this regard, base station 200 can include an RF data transmittercoupled to antenna 204. However, such RF data transmission circuitry isnot shown in the base station of FIG. 7, because in a simple embodimentof the technique, the E-field antenna 204 only broadcasts E-fields forthe purpose of charging the IPG's battery 26, as explained furtherbelow.

RF transmitter/receiver 166 can operate at a frequency, f_(E)′, that isdifferent from the transmission frequency of the E-field, f_(E).Choosing a different frequency for f_(E)′ can prevent interference withthe E-field 302 broadcast from the base station, and may allow for datareception at the base station E-field antenna 204 which is simultaneouswith such broadcast. If a different frequency f_(E)′ is chosen for backtelemetry, it may be advisable that such frequency not differ greatlyfrom f_(E); if an E-field at frequency f_(E) is successfully received atthe IPG 100, then it would be likely that transmission at a slightlydifferent frequency f_(E)′ would likewise be received at the basestation 200 without significant attenuation, etc. However, this is notstrictly necessary, and f_(E) can be significantly different fromf_(E)′. Alternatively, E-field 302 and transmission of data from the RFtransmitter/receiver 166 can be time multiplexed, in which case f_(E)can equal f_(E)′.

Having described the charging circuitry of both base station 200 and IPG100, discussion now turns to describing an exemplary method for chargingthe IPG 100 using the base station 200. In this example, the basestation 200 automatically produces a charging field when turned on, andin particular, microcontroller 212 (FIG. 7) initially selects B-fieldcharging as default. Using a B-field 304 as default means of charging ispreferred if it can be accomplished, because it can generally providemore energy to the IPG 100, and hence can charge the battery 26 faster.Therefore, microcontroller 212 outputs K1=1 to set the base station forB-field charging: i.e., to set the signal generator 214 to output afrequency of f_(B)=80 kHz; to activate the switch 222 to couple thetransmission circuitry to the coil 206; and to enable LSK receiver 220.At this point, the base station 200 is broadcasting the B-field 304,with the hope that an IPG 100 will receive this broadcast, and willacknowledge receipt by broadcasting either some sort of acknowledgment,or the charging information discussed earlier. Accordingly,microcontroller 212 waits for a certain period of time (e.g., oneminute) to receive back telemetry data at the LSK receiver 220. Duringthis “B period,” the base station 200 can adjust the strength of theB-field 304 via the gain control signal with the hope of producing aB-field 304 that will eventually be large enough to be recognized by theIPG 100. For example, the base station may start with gain control atits smallest setting, and ramp the gain until it reaches a maximum levelnearer to the end of the B period.

If IPG 100 is within range of the base station 200, its charging coil 18(FIG. 8A) will receive the B-field 304. Assuming the B-field receptionis strong enough, i.e., if V_(DCB)>V_(thB), input X to the IPG'smicrocontroller 158 will be asserted. As discussed earlier,microcontroller 158 then acknowledges that B-field charging hascommenced, and will set up the IPG 100 for charging by assertion ofcontrol signal B, which will enable charging circuitry 156 to choseV_(DCB) as its input, and enable LSK transmitter 160. At that point, andas is typical in IPGs configured for B-field charging, the LSKtransmitter 160 will start telemetering charging information (V_(BAT);coupling data, etc.) back to the base station 200 via coil 18. Suchcharging information produces reflection in the base station's coil 206,and is decoded at the LSK receiver 220. Receipt of such data (or someother form of acknowledgment) informs the base station 200 that the IPG100 is receiving the transmitted B-field 304, and that the base station200 should stay in B-field default mode by continuing to assert K1=1.Moreover, the base station 200 can begin to interpret the receivedcharging information, and modify the produced B-field 304 as necessary,i.e. by changing its magnitude via the gain control signal, and/or bychanging its duty cycle. See, e.g., the above-incorporated '733application.

If the battery 26 is fully charged, microcontroller 212, based on thereported value of V_(BAT), can cease generating the B-field. At thispoint, the base station 200 can default to E-field charging, asdiscussed further below. Providing lower-power E-field charging can bebeneficial should the IPG 100's battery 26 start to drain during use. Ifthe IPG 100 however will not benefit from E-field charging because itsbattery 26 is full, it can simply disable charging circuitry 156 forexample.

If the IPG 100 goes out of the range of the base station 200 or wasnever within range to start with, input X in the IPG 100 will equal ‘0’.As a result, microcontroller 158 in the IPG 100 will not acknowledgereceipt of a B-field (or an E-field at this point), and so controlsignals B and E will be disabled, such that IPG 100 will not send anyform of acknowledgement back to the base station 200. Eventually, e.g.,once the one-minute B period has expired, the microcontroller 212 in thebase station 200 will conclude that B-field charging cannot beaccomplished, and will now default to E-field charging. Accordingly,microcontroller 212 now asserts K1=0, which sets the signal generator214 to output a frequency of f_(B)=300 MHz; activates the switch 222 tocouple the transmission circuitry to the antenna 204; and enables RFreceiver 228.

In one embodiment, the base station 200 at this point will simplycontinue broadcasting the E-field 302 so long as it is powered on andwithout any communication from the IPG 100, that is, regardless ofwhether the IPG 100 can acknowledge and use the E-field for charging.This embodiment can be viewed as a simple, passive way to provideE-field charging: i.e., the low-power E-field 302 is produced, and it ishoped, but ultimately unknown, whether the E-field is of use to the IPG100. Such an embodiment is simple, because it doesn't require anycommunications from the IPG 100 to the base station. Hence, RFtransmitter/receiver 166 in the IPG 100, and RF receiver 228 in the basestation 200, can be dispensed with. However, because this communicationroute is useful and provides additional flexibility in tailoring thegenerated E-field, it is discussed further below.

If the IPG 100 is within range of the E-field 302, but outside of therange of the B-field 304, signal input Y at the IPG's microcontroller158 will be set to ‘1,’ assuming E-field reception is strong enough,i.e., if V_(DCE)>V_(thE). At that point, microcontroller 158 will set upthe IPG 100 for charging by assertion of control signal E, which willenable charging circuitry 156 to chose V_(DCE) as its input, and enableRF transmitter/receiver 166. The battery 26 will then begin charging,but as discussed above at a slower rate due to the relatively smallvalue of V_(DCE). RF transmitter/receiver 166 can then transmit thecharging information back to the base station 200 via its antenna 150.At the base station 200, such charging information is received atantenna 204, decoded at RF receiver 228, and used appropriately by themicrocontroller 212. For example, the microcontroller 212 can use thecharging information to modify the strength of the generated E-field 302via the gain control signal for example. Alternatively, themicrocontroller 212 could suspend generation of the E-field 302 ifV_(BAT) informs that the battery 26 is fully charged. However, and asdiscussed above, in another embodiment, the base station 200 can simplycontinue to generate the lower-power E-field 200 even if the battery iscurrently fully charged, in the off chance that the battery 26 depletesand will eventually be able to use the E-field for charging once again.

Charging of a patient's IPG battery 26 by E-field is a significantbenefit due to its relatively long effectiveness (e.g., >1 m), and eventhough it is imparts a relatively low amount of power to the IPG 100,such power can still be put to use to recharge the battery 26 if thepatient is in the vicinity of the E-field, even in passing.

In a preferred embodiment, the base station 200 can periodically assesswhether B-field charging is available, and can switch to that mode ifso. This is reasonable because an IPG 100 initially outside of the rangeof the base station 200 may come within range, because the patient hasmoved, is now lying down in bed, etc. Accordingly, periodically, e.g.,every 15 minutes or so, the base station 200 can revert to the B perioddiscussed above: it can assert K1=1 for a period of time to enableB-field charging, and adjust the strength of the B-field 304 to see ifthe IPG 100 acknowledges B-field reception. If so, the base station 200can continue production of the higher-power B-field 304. If not, thebase station 200 can once again start generating the lower-power E-field302 at the expiration of the B period.

FIGS. 9 and 10 respectively show alternative embodiments for the basestation 200 and IPG 100, where the back telemetry is carried out usinghardware and a communication channel that is separate from those used tocharge the IPG. Thus, FIG. 9 shows the base station with an additionalantenna 232, and FIG. 10 shows the IPG having an additional antenna 13.In this example, the antennas 232 and 13 are shown as coils, andcommunicate by magnetic induction. This is convenient, and isconsiderate of legacy system design, because the antenna 13 alreadyexists in the IPG 100 and is traditionally used to communicate with anexternal controller 12 (FIG. 2) as discussed in the Background. Asexplained earlier, such communication between antennas 232 and 13 couldoccur using an FSK protocol, and thus FSK transceivers 221 and 124 areshown coupled to antennas 232 and 13. Such communication can bebi-directional, or one-way from the IPG 100 to the base station for thepurpose of telemetering the charging information. By using thepre-existing coil 13 in the IPG 100 to also communicate with the basestation 200 during charging, system functionality can be expandedwithout the need to modify existing communication circuitry in the IPG100. However, is it not strictly necessary to use the pre-existingcommunications coil 13 in the IPG, and instead a separate dedicated RFor magnetic-induction antenna could be added to the IPG 100 and basestation 200 instead. Because this means of transmission between the IPG100 and the base station is not tied to the communication channels usedfor charging, note that the FSK transceivers 221 and 124 can be enabledusing control signals (FSK) having no connection to the control signalsused in the base station 200 or IPG 100 indicative of whether thosedevices are operating in a B-field or E-field mode (i.e., FSK isindependent of control signal K1 in the base station, or control signalsB and E in the IPG 100).

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An implantable medical device, comprising: a battery; a first antenna configured to receive an electric field for charging the battery; a second antenna configured to receive a magnetic field for charging the battery; and a microcontroller for determining whether either or both of the electric field and the magnetic field have been received at the first or second antennas respectively, and for issuing at least one control signal enabling charging of the battery either by the electric field or the magnetic field.
 2. The device of claim 1, further comprising a first rectifier coupled to the first antenna for producing a first DC voltage, and a second rectifier coupled to the second antenna for producing a second DC voltage.
 3. The device of claim 2, wherein the microcontroller determines whether either or both of the electric field and the magnetic field have been received at the first or second antennas respectively using first and second DC voltages.
 4. The device of claim 2, further comprising charging circuitry, wherein the charging circuitry is responsive to the at least one control signal to select either the first DC voltage or the second DC voltage as the power source for charging the battery.
 5. The device of claim 1, wherein the at least one control signal enables a transmitter for transmitting charging information from the implantable medical device.
 6. The device of claim 5, wherein the transmitter transmits the charging information from either the first or second antenna.
 7. The device of claim 1, wherein the charging information comprises a voltage of the battery.
 8. The device of claim 1, wherein the first antenna comprises an electrode lead coupled to an electrode for providing stimulation to a patient's tissue.
 9. An implantable medical device, comprising: a battery; a first antenna configured to receive a first field for charging the battery, wherein the first field is less than or equal to 100 kHz; a second antenna configured to receive a second field for charging the battery, wherein the second field has a frequency ranging from 1 MHz to 10 GHz; and a microcontroller for determining whether either or both of the first field and the second field have been received at the first or second antennas respectively, and for issuing at least one control signal enabling charging of the battery either by the first field or the second field.
 10. The device of claim 9, further comprising a first rectifier coupled to the first antenna for producing a first DC voltage, and a second rectifier coupled to the second antenna for producing a second DC voltage.
 11. The device of claim 10, wherein the microcontroller determines whether either or both of the first field and the second field have been received at the first or second antennas respectively using first and second DC voltages.
 12. The device of claim 10, further comprising charging circuitry, wherein the charging circuitry is responsive to the at least one control signal to select either the first DC voltage or the second DC voltage as the power source for charging the battery.
 13. The device of claim 9, wherein the at least one control signal enables a transmitter for transmitting charging information from the implantable medical device.
 14. The device of claim 13, wherein the transmitter transmits the charging information from either the first or second antenna.
 15. The device of claim 9, wherein the charging information comprises a voltage of the battery.
 16. The device of claim 9, wherein the first antenna comprises an electrode lead coupled to an electrode for providing stimulation to a patient's tissue.
 17. The device of claim 9, wherein the first field comprises an electric field, and wherein the second field comprises a magnetic field. 